1. Field of the Invention
This invention relates to a tooling system and method for repairing an irradiated fuel assembly wherein an instrument tube, fuel rod or other component must be removed. It is capable of removing such a component without the need for taking the fuel assembly apart.
2. Description of the Prior Art
Systems for removing components from irradiated fuel assemblies are known in the prior art. An example of such a system is described in U.S. patent application Ser. No. 746,897 filed June 20, 1985, by Anoop Kapoor et al. and assigned to the Westinghouse Electric Corporation. Generally, this application discloses a remote repair system for a nuclear fuel rod assembly that is capable of selectively withdrawing or inserting a fuel rod into or out of the lateral support grids of the assembly. However, before this particular system may be utilized, the top nozzle of the irradiated fuel rod must be removed from the rest of the assembly. This is accomplished by inserting an internal-diameter cutter into the open ends of the thimble tubes which connect the top and bottom nozzles together, and cutting these tubes from the inside at a point just beneath the top nozzle of the fuel rod assembly.
Unfortunately, the remote removal, storage, and reinstallation of a top nozzle assembly onto an irradiated nuclear fuel rod assembly requires the use of an expensive and sophisticated tooling system, and may only be successfully accomplished with the expenditure of a significant amount of time and effort. While such time and effort may be completely justified in a case where components are to be removed or installed, such time and effort is difficult to justify in a case where only one component in a fuel rod assembly is involved. One proposed solution to the problem of removing or installing a single component from a fuel rod assembly has been the use of a device that operates like a remote drill press and bores a hole in the top nozzle over the particular component involved to provide access thereto by means a gripping device. However, such a single-component removal system generates relatively large pieces of metallic debris and shavings which can become lodged in the spaces between the rods and the spacing grids that surround them, ultimately causing them to break through a fretting action. But before one can fully understand exactly how such metallic debris can cause these rods to weaken and break, some brief background as to the structure, operation and environment of such fuel rod assemblies is necessary.
Nuclear fuel rod assemblies generally comprise 200 to 290 fuel rods mounted in a square array within a support skeleton. The support skeleton in turn is formed from the aforementioned bottom and top nozzle assemblies which are connected to one another by twenty-four circularly arrayed thimble tubes. The bottom and top nozzles are about eight to nine inches square, and the thimble tubes are about thirteen feet long, so that the overall shape of the fuel assembly is that of an elongated, rectangular prism (see FIG. 1A). The fuel rods themselves are about twelve feet long. In order to equidistantly space the long and relatively flimsy fuel rods within the support skeleton, the skeleton includes approximately seven grids, each of which has a square array of apertures for receiving and spacing the fuel rods. The grids are usually sheet-metal structures fabricated from a heat-treated, high strength stainless steel in an "egg carton" configuration that lends compressive strength to the grids with a minimum amount of weight. In operation, after an array of nuclear fuel rod assemblies has been placed in the reactor core and irradiated, a jet of pressurized water is guided through the bottom nozzles thereof in order to uniformly absorb the heat generated by the rods. In nuclear reactors of the type designed by the Westinghouse Electric Corporation, the velocity of the pressurized water forced through the bottom nozzles of the fuel structures is on the order of fifteen feet per second. In some nuclear cores, this fifteen feet per second flow of water has created pressure differentials which in turn have resulted in side currents that flow laterally through the fuel rod assemblies disposed in the core. These side currents sometimes produce vibrations in the fuel rods. If sharp-edged, large particles of metallic debris fall and accumulate in the spaces between the rods and the cells defined by the grids of the fuel rod assemblies, this metallic debris can scour, puncture and eventually break the walls of the fuel rods as they laterally vibrate within the cells of the grids.
While the problems created by such metallic debris could perhaps be obviated by the provision of a suction means around the drill bit which could remove the particles as they are created by a vacuum-type action, the provision of such suction devices is expensive and cumbersome. Clearly, what is needed is a tooling system capable of providing an aperture in the adapter plate without the production of metallic chips, particles and other debris which can lodge in the spaces between the grid cells and the rods and ultimately cause the rods to break due to the fretting of the rods against the particles. Ideally, such a system should be remotely operable since irradiated fuel assemblies create an intensely radioactive environment, but also simple, reliable, and relatively inexpensive.